Process for electrolytically regenerating ferric chloride etching solutions



y 23, 1963 E. s, DOUGLAS 3,394,060

PROCESS FOR ELECTROLYTICALLIY REGENERATING FERRIC CHLORIDE ETCHINGSOLUTIONS Filed Feb. 19, 1965 I /4a ..-I

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United States Patent 3,394,060 PROCESS FOR ELECTROLYTICALLY REGEN-ERATING FERRIC CHLORIDE ETCHING SOLUTIONS Ellwood S. Douglas, 1429Oregon Blvd, Berkeley, Calif. 947% Filed Feb. 19, 1965, Ser. No. 434,0306 Claims. (Cl. 204--94) This invention relates generally to etchingsolutions and more particularly to a process for regenerating spentferric chloride solutions utilized when etching metal such as copper,nickel, etc.

In the manufacture of printed circuits, for example, copper ischemically removed by dissolving it in an aqueous solution of anoxidizing agent. One of the more desirable etching solutions used toetch copper is an aqueous solution of ferric chloride. The ferricchloride solution is generally supplied and used as an aqueous solutionat a concentration of about 3.4 molar or about a density of 42 Baum. Theferric chloride is converted to ferrous chloride during the etchingreaction as can be expressed by the equation:

As the concentration of chlorides of the etched metal in the solutionincreases, and the concentration of ferric chloride decreases, the timerequired to etch a given amount of metal greatly increases and, itbecomes uneconomical to continue using the etching solution.

The relatively expensive ferric chloride is generally purchased as a 42*Baum solution, although a comparable solution may be made up byutilizing ferric chloride crystals. This etching solution is generallyused until the time required to etch a given amount of metal has doubledover that of fresh solution. The solution at that point would containabout FeCl FeCl 1.9M CuCl 1.0M

It may therefore be seen that where large quantities of spent etchingsolution are involved, it is not economical to discard the spentsolution due to its inherent corrosiveness and its content of valuablemetallic salts. Accordingly, attempts have been made to economicallyrecover the metallic salts and regenerate the etching solution to asubstantially fresh condition sothat it may be reused in the etchingprocess, however, the prior attempts to accomplish this economicallyhave not been entirely successful.

It is therefore an object of this invention to set forth an improvedprocess for regenerating spent ferric chloride etching solution.

It is another object of this invention to set forth a process foreconomically recovering the valuable etched metal from a spent ferricchloride etching solution.

It is still another object of this invention to set forth an improvedprocess for regenerating a spent ferric chloride etching solution by anovel organic solvent extractionconcentration-electrolytic process.

A further object of this invention is to set forth a process forcontinuously regenerating a solution used for the chemical etching ofmetals.

3,394,060 Patented July 23, 1968 iCe For purposes of illustration, theinstant process will be (a) ZFeCl +Cu=CuCl ZFeCl The process for theregeneration of ferric chloride and the recovery of copper from thespent etching solution may be broadly described as a solventextraction-electrolytic process. The ferric chloride remaining in thespent etching solution is removed by solvent extraction leaving FeCl andCuCl in solution. This solution is electrolyzed in a cell with apolarized cathode. The half reactions are at the cathode:

At low cathode currents only half reaction (c) occurs. When the currentis increased beyond a value corresponding to reduction of all Fe+++ ionswhich diffuse to the cathode, voltage increases, and half reaction (d)occurs. Similarly, when current exceeds that given by reduction of allFe+++ and all Cu++ ions which diffuse to the cathode, half reaction (e)begins. Likewise, when the current exceeds the value corresponding tothe reduction of all Fe+++ to Fe++, and all Cu++ to Cu, voltage againincreases causing half reaction (f). Half reaction (0) is undesirable;it is the reverse of half reaction (g) which occurs at the anode.Current which flows due to half reaction (c) represents a waste ofpower. One of the reasons for solvent extraction of FeCl is to maintainthe Fe+++ concentration appreciably lower than the Cu++ concentration.Most of the current then participates in the useful half reactions (d)and (e), and very little is wasted in half reaction (c). The current dueto reaction (c) is limited by the diifusion of Fe+++ to the cathodesurface and diffusion is approximately proportional to concentration.Evolution of H by half reaction (h) is also undesirable. The evolutionof H in this manner wastes power and may be hazardous. Under controlledconditions, however, the evolution of H produced by half reaction (h)may be monitored for the automatic control of the current density toassure completion of half reaction (e).

It is desirable that all Cu++ ions diffusing to the cathode and reducedto Cu+ are subsequently reduced to Cu. If Cu" ions escape from thevicinity of the cathode, they may encounter the anode and be oxidizedback to Cu++ by half reaction (h). This again represents a waste ofpower. The desired reaction at the anode is (g). Alternate orsimultaneous solvent extraction is used to remove Fe+++ as it isproduced by half reaction (g). This minimizes the wasteful half reaction(c) as explained earlier.

The solvent extraction of the feric chloride is carried out at low pH.The distribution of Fe+++ between equal volumes of water and organicsolvent such as ethyl ether or ethyl acetate, for example, inequilibrium contact is a function of R concentration in the watersolution. At 0.1 molar H most of the Fe is in the water solution: at 3molar H about half is in each phase. At 6 molar H 99% of the Pe+++ is inthe organic solvent. It is therefor necessary to maintain high acidityin order to remove Fe effectively by organic solvent extraction, so thatthe electrolytic reduction of Cu+ to Cu can be efiiciently accomplished.If the organic solvent used is one which boils at a lower temperaturethan water, it may be boiled by contact with the hot water, and thesolvent vapor condensed for reuse. The ferric chloride extracted isaccordingly left in the hot water. This mixture of ferric chloride andhydrochloric acid in water gives up its hydrochloric acid upon boiling.Upon additional boiling, fenric chloride hydrolyzes to Fe(OH) giving upstill more HCl as vapor. If the amount of heat applied is controlled bymonitoring the pH to leave a slight excess of acid, and water added togive a density of 42 Baum (39% FeCl the resulting solution will beidentical to fresh ferric chloride etching solution.

The HCl vapor given off is returned to the solution being reclaimedWhere it is needed to augment the extraction of more Fe+++.

In order to keep the electrolytic cell to a reasonable size, and assureeconomical operation, it is necessary to electrolyze solutions of highFeC1 -CuCl -HCl concentration. To prevent dilution, as little Water aspossible should be in the input or some means of removing water must beprovided. The requirement of high H+ concentration for effective solventextraction of the Fe+++ also demands control of dilution. Dehydration ofthe input results in hydrolysis and the loss of some HCl. A solid inputof spent solution would therefore have to be gathered after dehydrationand handled as a solid, which is more difficult than handling a liquid.These reasons make it more desirable to remove water from theelectrolyte than to dehydrate the input. This can be done by boiling. Itshould be done at the point where the system has the lowest acidity,since HCl will also boil out. Water and HCl form a maximum boilingazeotr-ope of HCl, 80% Water. This is approximately 6 molar HClsolution. Solutions Weaker in HCl boil off water and approach theazeotropic concentration. Some HCl vapor escapes under these conditions:Boiling in a fractionating still with a reflux of pure water will allowconcentration from 3 molar to nearly 6 molar with an output of steam atthe top which contains little HCl.

The figure is a flow sheet of the instant process for regenerating spentferric chloride etching solution, and recovering the etched metalcontained therein.

The system shown schematically in the figure utilizes ethyl ether toextract ferric chloride from the aqueous solution, however, it is to beunderstood that many oxygen containing organic compounds could beutilized in place of ethyl ether in a system of this nature. Alternativesolvents would include such compounds as ethyl acetate, isopropyl ether,n-butyl acetate, higher alcohols, aldehydes, ketones, etc., for example.For a better understanding of the instant process illustrative flowrates and flow compositions are given although it is to be understoodthat they do not necessarily represent optimum values.

Spent ferric chloride etching solution is withdrawn from the etchingequipment through line 13 and is accumulated in a spent etching solutionreservoir 1. The composition of the spent solution is assumed to be asfollows:

The spent solution is withdrawn from the reservoir 1 by means of flow14, and mixed with a return flow 22, which is from a succeeding stage ofthe system, as will be described later, and enters a primary solventextraction contactor 2. Approximate flow rates elsewhere in the systemare based upon a flow of approximately one gallon per unit time at 14.

The composition of flow 22 is about:

FeCl 0.1M FeCl 2.0M CuCl 1.0M HCl 5.0M

and is about three gallons.

Therefore about four gallons of aqueous solution of the composition:

FeCl 0375M FeCl 2.0M cuci 1.0M HCl 3.75M

F6C13 FeCl 2.0M cucl 1.0M HCl 3.4M

The aqueous flow 15 enters a regenerative heat exchanger 3 at about 25C. and emerges as How 16 at about C. A vapor trap 16a is placed in flow16 to prevent the loss of ether vapor due to ether dissolved onentrained in flow 15. The trapped vapor is fed through flow 16b to anether condenser 9, the complete function of which will be describedlater.

The fiow 16 is fed into a fractionating column 4. The fractionatingcolumn receives one gallon of distilled water reflux at the top asrepresented by flow 19. The column exhausts steam as flow 20. The steamis equivalent to two gallons of water, giving a net removal of onegallon of water at the top. Flow 20 contains less than 0.1M HCl. A partof flow 14 as indicated by flow 14a may be used in place of water atflow 19 to absorb HCl gas from the steam in the fractionating column.Heat input flow 18 operates the fractionating column and represents themajor use of fuel in the system. Flow 20, as indicated by the flow shownas a broken line 20a, may be used to provide the heat, and water flowrequired by an ether boiler 8, the complete function of which will bedescribed later.

The flow 17 leaving the fractionating column 4 is about three gallons ofan aqueous solution of the following composition:

FeCl 0.15M F6012 2.66M cuci 1.33M HCI 5.0M

The flow 17 enters the regenerative heat exchanger 3 at about C. andleaves the heat exchanger 3 as flow 21 at about 30 C. The flow 21 is fedto an intermediate solution regeneration reservoir 5 whose contents havea composition the same as flow 22 previously described. A portion of thesolution reservoir 5 is recycled through flow 22 to the primaryextractor 2 as previously described.

Flow 23, of about thirty gallons, removes solution from the intermediatereservoir 5 and passes it through an electrolytic cell 6 where (u isreduced to copper and deposited out at the cathode and Fe++ is oxidizedto 5 Fe+++ at the anode. The electrolytic cell 6 is of conventionaldesign and includes cathodes of suitable mate rial upon which copper maybe accumulated and removed from the cell in the form of a foil, forexample. As stated previously, the current input to the cell is suchthat the desirable half reactions:

at the cathode are:

(e) Cu++e-=Cu at the anode:

The high flow rate through the electrolytic cell 6 pro vides sufiicientagitation for the electrolyte.

A secondary organic solvent extraction is used simultaneously to removeFe+++ as it is produced by the half reaction (-g) in the electrolyticcell 6. This minimizes the wasteful half reaction (c) Fe++++e- =Fe++ asexplained earlier.

Accordingly, a portion of the electrolyte from cell 6 is returned to theintermediate reservoir 5 by flow 24 and a portion of the electrolyte isfed by flow 25 to the secondary organic solvent extraction contactor 7Where Fe+++ is removed by ether. The ether for extractors 2 and 7 issupplied by an ether condenser and reservoir 9. Additional organicsolvent may be added to the reservoir 9 to replace solvent lost. Theaqueous phase from the extractor 7 is returned by flow 26 to theintermediate reservoir 5 and the ether laden with FeCl HCl-H O from theextractor 7 is fed by flow 27 to the ether boiler 8. The ether boiler 8,also receives the ether phase, through flow 28, from the primary organicsolvent extractor 2. The ether laden wtih FeCl -HCl-H o returns by flows27 and 28 to the ether boiler 8-, where the ether is evaporated. Theether vapor passes by flow '30 back to the ether condenser 9 where it iscondensed for reuse in the extractors 2 and 7.

Water flow 29 and heat input 33 are added to ether boiler 8 to evaporatethe ether. As shown by the flow 20a (in broken lines) the water and heatrequired by the ether boiler 8 may be derived from the output of thefractionating column 4. The heat flow 33 is needed to evaporate theether, and the water flow 29 to prevent the residue Fec-l -HCl-nH O fromprecipitating solid FeCl The aqueous solution represented by flow 34contains FeCl and HCl. Flow 34 is fed to a boiler 10 which is heated bya heat input 39 which evaporates H and HCl so that flow 37 has a higherFeCl concentration than desired in the final regenerated ferric chloridesolution. The HCl and H 0 vapor flow 35 from the boiler 10 is liquifiedin condenser 12 and returned by flow 36 to the intermediate reservoir 5.

Flow 37 is fed to a regenerated etching solution reservoir 11 wheredistilled water is added to adjust the re generated solution to 42 Baum(3.4M). The regenerated ferric chloride solution is then Withdrawn byflow 38, as required, to the etching equipment.

The ether may also be evaporated by contacting the flows 27 and 28 whichare ether laden with FeCl HCl -H O with molten FeC1 withdrawing the FeClenriched molten FeCl and subsequently diluting the FeCl with water tosubstantially the same concentration as fresh etching solution. Whenutilizing molten FeCl to evaporate the ether and HCl the ether boiler 8and boiler 10 would be eliminated and a molten FeC1 contactor 10a wouldbe substituted in their place. Molten FeCl at about l50-175 C. would befed into the contactor 10a by flow 10b. The flows 27 and 28 would be fedto contactor 10a by the broken line flow 27a. The ether and acid vaporsare fed to the condensers 9 and 12 by the broken line flows 30a and 35a.The enriched molten FeCl is fed from the contactor 10a to theregenerated etching solution reservoir 11 by the broken line flow 37awhere distilled water is added to adjust the regenerated solution to 42Baurn (3.4M).

It may, therefore, be seen that a highly efficient process has beenprovided for the economical regeneration of the etching solution andrecovery of the valuable etched metallic salts contained therein,thereby great-1y reducing the cost of the etching solution required. Theinstant solvent, extraction-concentration-electrolytic process providesan economy of operation not possible heretofore.

From the foregoing illustration and specific example it will be apparentthat I have provided a new and improved process for regenerating spentferric chloride etching solution. Accordingly I believe that myinvention should not be necessarily limited to the specific example setforth, but rather should be determined by the claims appended below.

What is claimed as new is as follows:

1. In the process of etching a metal with an aqueous ferric chlorideetching solution in which process a portion of the ferric chloride isreduce-d to ferrous chloride and the metal being etched is oxidized to ametallic chloride, the improved process of recovering the metal etchedand regenerating the spent etching solution to a substantially freshcondition, comprising the steps of acidifying the spent solution, makinga first extraction of residual fenric chloride with the aid of anorganic solvent selected from the group consisting of etyhl ether, ethylacetate, isopropyl ether, n-:butyl acetate, higher alcohols, aldehydesand ketones, concentrating the remaining aqueous solution laden withferrous chloride, metallic chloride of the metal being etched and acid;electrolytically oxidizing the ferrous chloride to ferric chloride andreducing the metallic chloride of the metal being etched to elementalmetal while simultaneously making a second extraction of ferric chloride so .produced, evaporating the organic solvent from the organicsolvent phase of said first and second extraction steps, boiling offexcess acid thereby producing a regenerated etching solution.

2. The process of claim 1 wherein the organic solvent and acid vaporsare liquified for reuse in the process.

3. In the process of etching a metal with a ferric chloride etchingsolution in which process a portion of the ferric chloride is reduced toferrous chloride, the improvement comprising the steps of regeneratingthe spent etching solution by first extracting substantially all of theresidual ferric chloride from said spent etching solution with anorganic solvent selected from the group consisting of ethyl ether, ethylacetate, isopropyl ether, nbutyl acetate, higher alcohols, aldehydes andketones in the presence of sufiicient hydrochloric acid to assuresubstantially complete extraction of the residual ferric chloride;partially dehydrating the remaining aqueous solution containing ferrouschloride and the chloride of the metal being etched; subjecting saidferrous chloride and said metal chloride to electrolysis in the presenceof hydrochloric acid to oxidize the ferrous chloride to ferric chlorideand reduce the ecthed metallic chloride to recover the elemental metalwhile simultaneously extracting ferric chloride so produced therebyregenerating the etc-hing solution.

4. In the process of electrolytically regenerating a spent aqueousferric chloride etching solution comprising ferric chloride, ferrouschloride, and a metallic chloride of the metal etched, the improvementcomprising the steps of extracting residual ferric chloride from thespent etching solution and simultaneously extracting the ferric chlorideelectrolytically produced with the aid of an organic solvent andseparating said organic solvent to produce a ferric chloride etchingsolution of substantially the same composition as fresh etchingsolution.

5. The process of claim 4 wherein the organic solvent is boiled off bycontact with hot water, thereby precluding the necessity of collectingthe ferric chloride as a solid.

6. In the process of electrolytically regenerating a spent aqueousferric chloride etching solution comprising ferric chloride, ferrouschloride, and a metallic chloride of the metal etched, the improvementcomprising the steps of extracting residual ferric chloride from thespent etching solution and simultaneously extracting the ferric chlorideelectrolytically produced with the aid of an organic solvent, boilingoff said organic solvent by contact With molten ferric chloride anddiluting the residual solution with water to produce a ferric chlorideetching solution of substantially the same composition as fresh etchingsolution.

References Cited UNITED STATES PATENTS 1,197,556 9/1916 Slater 20494 51,451,333 4/1923 Eustis et a1 2041l3 2,828,194 3/1958 Hopkins et a1252-79.2 2,964,453 12/1960 Garn et al 20494 JOHN H. MACK, PrimaryExaminer.

10 H. M. FLOURNOY, Assistant Examiner.

1. IN THE PROCESS OF ETCHING A METAL WITH AN AQUEOUS FERRIC CHLORIDEETCHING SOLUTION IN WHICH PROCESS A PORTION OF THE FERRIC CHLORIDE ISREDUCED TO FERROUS CHLORIDE AND THE METAL BEING ETCHED IS OXIDIZED TO AMETALLIC CHLORIDE, THE IMPROVED PROCESS OF RECOVERING THE METAL ETCHEDAND REGENERATING THE SPENT ETCHING SOLUTION TO A SUBSTANTIALLY FRESHCONDITION, COMPRISING THE STEPS OF ACIDIFYING THE SPENT SOLUTION, MAKINGA FIRST EXTRACTION OF RESIDUAL FERRIC CHLORIDE WITH THE AID OF ANORGANIC SOLVENT SELECTED FROM THE GROUP CONSISTING OF ETHYL ETHER, ETHYLACETATE, ISOPROPYL ETHER, N-BUTYL ACETATE, HIGHER ALCOHOLS, ALDEHYDESAND KETONES, CONCENTRATING THE REMAINING AQUEOUS SOLUTION LADEN WITHFERROUS CHLORIDE, METALLIC CHLORIDE OF THE METAL BEING ETCHED AND ACID;ELECTROLYTICALLY OXIDIZING THE FERROUS CHLORIDE TO FERRIC CHLORIDE ANDREDUCING THE METALLIC CHLORIDE OF THE METAL BEING ETCHED TO ELEMENTALMETAL WHILE SIMULTANEOUSLY MAKING A SECOND EXTRACTION OF FERRIC CHLORIDESO PRODUCED, EVAPORATING THE ORGANIC SOLVENT FROM THE ORGANIC SOLVENTPHASE OF SAID FIRST AND SECOND EXTRACTION STEPS, BOILING OF EXCESS ACIDTHEREBY PRODUCING A REGENERATED ETCHING SOLUTION.